Abyssal Dungeon Entry lost due to group immediately leaving

I joined a group for Hard mode Oreha’s Well dungeons. After we got in, the group immediately all left and I was stuck inside. I was unable to click the quit dungeon button because it failed when they all left.

Then 30 seconds later, I was kicked out of the instance and lost my weekly entry. Why was I counted as a leaver if I didn’t leave, but the rest of the group did?